Polar vinyl polymers are vinyl polymers having polar pendant groups. They play an important role in industry and commercial processes. For example, polar vinyl polymers such as polyacrylonitrile, poly (vinyl alcohol), poly (methyl methacrylate) and polyacrylamide are well known and frequently used in industrial processes. Theoretically, the control of the microstructure of polar vinyl polymers can provide control of the qualities and properties of the polymers. Some of the properties of polar vinyl polymers that need to be controlled include stereoregularity, molecular weight, chain-end and monomer distribution.
Polar vinyl polymers are generally prepared by polymerizing monomers using a radical polymerization process. Radical polymerization processes are highly versatile and are commonly used in industry to prepare polymers. However, there is little regularity of the chemical structure of polymers obtained according to conventional processes, especially when polymerizing polar vinyl monomers. When these polymers are molded and shaped into fiber, film, or molding, the mechanical, chemical and thermal characteristics are not always satisfactory because the polymer crystallinity is low. For example, in the case of polyacrylonitrile, a fiber or film having low crystallinity shows poor heat resistance, especially in the wet state. Hence, the development of high-grade fabric articles with such polymers is limited and their use in many industrial or space applications is similarly limited.
In the case of non-polar polyolefins, stereoregular polymerization has been accomplished using heterogeneous or homogenous organometalic catalysts such as Zieglar-Natta, metallocene, or late-transition metal complexes during polymerization. Most of these catalysts, however, are not effective for the polymerization of polar vinyl monomers because the polar group in the monomer deactivates the catalyst. A few catalysts such as rare earth metal complexes (J. Am. Chem. Soc. 1992, 114, 4908, Macromolecules 1996, 29, 8014), have been successfully employed for the stereospecific polymerization of methacrylates, but even with these catalysts, other polar vinyl monomers such as acrylonitrile can not be polymerized, let alone in a stereoregular fashion. Furthermore, some of these catalysts are highly unstable in the presence of air and moisture, which has inhibited their development in industrial polymer production.
Anionic polymerization has also been reported as an effective method for stereoregular polymer production. Anionic polymerization, however, is also industrially impractical due to low efficiency (process and/or yield) and low molecular weight of the resistant polymer due to sensitivity of the intermediates and side reactions that occur between the initiator or propagation anionic species and the polar group. For example, the reaction of tert-butyllithium/trialkylaluminum initiator and alkylaluminum/phosphin complex reportedly produces syndiotactic methacrylates, but the resultant polymers formed by this reaction have low molecular weights of less than 10,000. Isotactic polyacrylonitrile with meso-meso (mm) triad up to 70% was obtained when acrylonitrile was polymerized using alkyl alkaline earth compounds such as Mg as the initiators at 70° C. Unfortunately, significant side reactions between the cyano group and the initiator restricts polymer yields to a range near 10–30%.
Several studies have focused on the stereo-controlled free radical polymerization of vinyl monomers. If the monomers are absorbed into several inclusion compounds such as urea-acrylonitrile and deoxycholic acid-methacrylates, and irradiated by X-ray radiation at low temperatures, polymers having the stereoregularity, i.e., isotactic-rich polyacrylonitrile or syndiotactic-rich poly(methyl methacrylate) are obtained (i.e., J. Am. Chem. Soc., 1960, 2, 5671). Recently, isotactic or syndiotactic poly (vinyl alcohol) has been prepared by radical polymerization of bulky vinyl esters in the presence of bulky fluoroalcohols that are thought to complex to the monomers via hydrogen bonding. Saponification is then necessary in order to deprotect to the alcohol (Macromolecules, 1999, 31, 7598). These systems, however, are still not appropriate for industrial applications due to the specificity of host-monomer combination and the conditions of the polymerization process (e.g., necessity of low temperatures below −30° C.) and/or photo irradiation due to the instability of the hydrogen bonded complexes above 0° C.
Several decades ago, Lewis acids such as zinc chloride had been found to significantly affect monomer reactivity and stereochemistry during radical copolymerization. However, the stereochemistry during radical homopolymerization with Lewis acid had not been reported until very recently because no stereoregularity had been observed. In 2001, it was reported that use of catalytic amount (0.1–0.2 equivalents per monomer) of specific rare metal containing Lewis acids such as scandium trifluoromethanesulfonate (Sc(Otf)3) was shown to effect on the stereospecific radical polymerization of some polar vinyl monomer such as acrylamide and N,N-disubstituted acrylamide (i.e., J. Am. Chem. Soc. 2001, 123, 7180). However, even with these specific rare metal Lewis acids, clear effect on the stereospecific polymerization of other conventional vinyl monomers such as methacrylates, acrylonitrile and vinyl acetate have not been shown. Moreover, these rare-earth metal Lewis acids are expensive and any improvement of the polymerization process must include the recycling of rare-earth metals, which is expensive and undesirable.
Therefore, processes and methods for producing stereoregular vinyl polymers from polar vinyl monomers are desirable.